The present invention relates to a method of color printing with an ink jet printer having a printhead which reciprocatingly moves in forward and return scan passes in a main scanning direction, in which an even number of nozzle arrays is provided for each of at least two colors, and the nozzle arrays are symmetrically arranged in the main scanning direction. The present invention further relates to an ink jet printer for carrying out this method.
The printhead of an ink jet color printer typically has at least one nozzle array for each color. For example, in three-color printing, three nozzle arrays will be provided for the colors cyan (C), magenta (M) and yellow (Y). Each nozzle array comprises a plurality of nozzles which, in case of a linear array, for example, are arranged equidistantly on a line that extends in a direction orthogonal to the main scanning direction. The distance between neighboring nozzles corresponds to the distance between adjacent pixels of the image to be printed, so that the number of pixel lines that can be printed in one scan pass corresponds to the number of nozzles in the array. The nozzle arrays for the various colors are sequentially arranged in the main scanning direction, for example in the order C-M-Y. In order to print a pixel that is composed of several colors, the corresponding nozzles of the various arrays are energized at appropriate timings, so that ink droplets jetted out from the various nozzles impinge on the recording sheet essentially at the same location, i.e. the location of the pixel to be printed. Thus, the ink droplets of different colors composing the pixel will be superposed one upon the other or will at least overlap with each other. During a forward scan pass of the printhead, the ink droplets of different colors will be superposed on the recording sheet in the order C-M-Y, and during a return stroke or scan pass of the printhead, the ink droplets will be superposed in reverse order.
It has been found however that the order in which the ink droplets are superposed one upon the other has an influence on the hue of the printed pixel. This effect is particularly marked for green colors composed of cyan and yellow ink. When a cyan droplet is superposed on a yellow droplet and both droplets have the same volume, then the cyan component forming the top layer will be predominant and the hue of the printed green pixel will be shifted to the blue side of the spectrum. Conversely, when the yellow droplet is superposed on the cyan one, then the hue will be shifted to the yellow side. As a result, when a solid green area is to be printed, the shifts in the hue will be visible to the human eye in the form of bands extending in the main scanning direction and having a width corresponding to the height of the nozzle arrays.
In order to eliminate this undesirable artefact, U.S. Pat. No. 4,528,576 discloses a printing method in which two nozzle arrays are provided for each color, and the nozzle arrays are symmetrically arranged in the main scanning direction, for example in the nested sequence C-M-Y-Y-M-C. When the nozzle head moves in forward direction, only the first three nozzle arrays are used, so that the colors are superposed in the order C-M-Y. In the return scan pass of the printhead, the other three nozzles heads are used, so that the colors are again superposed in the same order. This avoids the creation of bands with different hue but has the disadvantage that the nozzle arrays are utilized quite inefficiently, because only one half of the total number of nozzle arrays is used in each scan pass.
It is an object of the present invention to provide an ink jet color printing method and printer in which the hue of the printed colors does not depend on the direction of the scan pass and which at the same time permit high resolution and high density printing with efficient use of the nozzle arrays.
According to the present invention, this object is achieved by a method, in which, for printing a single pixel composed of at least two colors with high density, ink dots of each of these colors are formed in even numbers by activating the corresponding nozzles of the symmetrically arranged arrays at appropriate timings in the same scan pass of the printhead.
Thus, if a green pixel is to be printed, for example, then the pixel will be composed of at least four ink droplets two of which are formed by yellow ink and the other two of which are formed by cyan ink. The order in which these ink droplets are printed and superposed will, for example, be C-Y-Y-C, irrespective of the direction of the scan pass.
This method has the advantage that it permits a high printing resolution without degrading the image density of the printed image.
It is generally known that, when the printing resolution of an ink jet printer is increased, the volume of the ink droplet becomes so small that the density of the image may become insufficient. This can be understood from the following simplified consideration. As has been mentioned above, the size of the pixels to be printed, and hence the printing resolution, is determined by the pitch of the nozzles in the array. Each nozzle is connected to an ink channel or cavity in which the ink liquid is pressurized by means of a piezoelectric actuator or the like. Thus, the space available for the various cavities is limited by the pitch of nozzles. Further, the cross section of the nozzle orifice must be smaller than the cross section of the cavity in order to achieve a nozzle action which is necessary for forming the ink droplets. As a result, the maximum diameter of the nozzle orifice is roughly proportional to the pitch of the nozzles. If the printing resolution is to be increased by a certain factor r, then, roughly speaking, the whole nozzle array should be scaled down by the factor 1/r. The diameter of the ink droplet expelled from the nozzle is roughly proportional to the diameter of the nozzle orifice, so that the volume of the ink droplets will be decreased by a factor 1/r3. However, the surface area of the pixels to be printed will be decreased only by the factor 1/r2, so that the image density, i.e. the ink volume per unit surface area will decrease approximately with 1/r.
It is generally known that this decrease of the image density can be compensated for by employing a multi-pass system in which, even in a single color printer, a pixel to be printed is composed of two or more ink droplets which are generated in successive scan passes.
The present invention combines this principle for improving the image density with the known method of eliminating shifts of the hue in color printing and thereby makes more efficient use of the nozzle arrays which anyway must be provided at least two-fold for each color.
In a preferred embodiment, the printhead comprises two nozzle arrays for each color, and the nozzle arrays are arranged in two blocks, so that each block comprises a complete set of nozzle arrays for each color, and one block is the mirror image of the other.
Preferably, the nozzle blocks themselves are mirror-symmetric with respect to a median plane which intersects the linear nozzle arrays at right angles. Then, the two nozzle blocks may have an identical construction and may be manufactured in exactly the same procedure, and when the blocks are mounted to the printhead, one of them is mounted in an orientation rotated by 180xc2x0. This increases not only the production efficiency but has the further advantage that the effect of certain defects of the nozzle arrays may be mitigated. Such defects may, for example, consist of a slight misalignment of the nozzles within the nozzle arrays or of minor defects of the nozzle orifices which cause the ink droplets to be jetted out not exactly along the axis of the nozzle but under a slight screw angle. When the two nozzle blocks are obtained in the same production process, these defects will be essentially the same for both blocks, and when the two blocks are mounted on the printhead in mutually opposite orientation, the dot misplacements caused by these defects in the one nozzle block will be opposite to those caused in the other nozzle block, and by superposing these dots, the dot misplacements will be smoothened out.
In one embodiment, the individual pixels will be formed by exactly superposing the color dots one upon the other. In a modified embodiment, it is possible, however, to create elongated pixels by providing a slight offset between the first and the second dot of the same color, so that these dots still overlap but are not exactly congruent. As a result, the printing resolution will be unisotropic, which may be beneficial in certain applications as a compromise between the highest possible printing speed and reproduction of sufficient details of the printed image.
The offset between the dots may be either in the main scanning direction or in sub-scanning direction. In both cases, the offset can be brought about by a corresponding physical offset of the two nozzle blocks. If the offset is in the main scanning direction, it may alternatively be brought about by a corresponding shift between the timings at which the nozzles of the two blocks are energized.
With elongated pixels, it is also possible to operate the printer in a mode in which the two nozzle blocks are controlled independently from one another, so that the resolution in longitudinal direction of the pixels is increased.